Theresa May faces a humiliating defeat over Brexit as Labour warns it will block the so-called Repeal Bill if the prime minister fails to implement dramatic changes.

The government has on Thursday published the bill – deemed “one of the largest legislative projects ever undertaken in the UK” – saying the legislation will “maximize continuity” on the day the UK effectively withdraws from the EU wherever it is “practical and sensible”.

The bill seeks to repeal the 1972 European Communities Act so that EU law no longer has precedence over UK law.

It copies all EU law into UK legislation so that individuals and businesses are not frustrated as Brexit comes into effect, but Parliament will then be able to change “amend, repeal and improve” the laws as necessary.

Brexit Secretary David Davis said the bill would allow the UK to withdraw from the EU with “certainty, continuity and control,” adding it would comply with the British people’s will as it “ensures that the decisions that affect our lives are taken here in the UK.”

“It is one of the most significant pieces of legislation that has ever passed through Parliament and is a major milestone in the process of our withdrawal from the European Union,” he remarked, according to the BBC.

The government, however, is already being criticised over the bill which ditches the EU Charter of Fundamental Rights, something the Labour party has said would be a prerequisite for the opposition to pass the bill.

Labour has threatened to block the bill if six key amendments fail to be implemented.

Among the changes, Labour is calling for a guarantee that workers’ rights in the UK do not lag behind the EU’s, and that the widespread ‘Henry VIII’ powers allowing for government to amend the statute without Parliament’s approval are restricted.

Shadow Brexit Secretary Keir Starmer warned the party would seek to block the legislation within three months if a “right approach” to Brexit is not taken.

“Labour has always been clear that Brexit cannot lead to any rolling back of rights and protections.

“We need effective legislation that protects British workers and consumers, enshrines equality laws, enforces environmental standards and devolves powers across the country,” he said, according to the Independent.

“The government’s Repeal Bill falls short on all counts. It is simply not fit for purpose.”

He then remarked that despite the Tories’ majority being wiped out in the general election, the PM is yet to change her approach on Brexit so that it is more favorable to MPs across all parties.

“I haven’t seen any evidence that the prime minister has reflected on the outcome of the general election and indicated a willingness to change her approach to Brexit.

“On the contrary, she has reinforced the approach that she took to the electorate.”

Liberal Democrats leader Tim Farron also cautioned the government that the attempt to pass the bill through Parliament would be “hell.”

“If you found the Article 50 Bill difficult, you should be under no illusion, this will be hell,” he said.

“If the Government try any wheeze or trick to force through changes to vital protections, from workers’ rights to the environment, they are playing with fire.”